Modular biocular eyepiece for thermal image systems

ABSTRACT

A biocular viewing system for thermal (far infrared) viewers is provided by matching an eyepiece developed for an image intensifier to the low intensity image produced by light emitting diodes. A simplified image intensifier with gain sacrificed provides a low cost matching unit.

United States Patent 1 Anderson NOV. 25, 1975 [5 MODULAR BIOCULAR EYEPIECE FOR THERMAL IMAGE SYSTEMS David K. Anderson, Woodbridge, Va.

[73] Assignee: The United States of America as represented by the Secretary of the Army, Washington, DC.

221 Filed: Nov. 15, 1974 211 App]. No.: 524,288

Related US. Application Data [62] Division of Ser, No. 399,016, Sept, 20, 1973, Pat,

[75] inventor:

[52] US. Cl. 250/330; 350/175 E [51] Int. Cl. H013 31/49; H01] 31/50 [58] Field of Search 250/330, 332, 334; 350/175 E [56] References Cited UNITED STATES PATENTS 3,614,209 10/1971 Seaman 350/175 E 3,658,412 4/1972 Seaman 1. 350/175 E 3,764,194 10/1973 Trowbridge et al... 350/175 E 3,812,357 5/1974 Flogaus et a1 4. 250/330 Primary Examiner-James W, Lawrence Assistant ExaminerT. N, Grigsby Attorney, Agent, or Firm lohn E, Holford; Nathan Edelberg; Robert P. Gibson [57] ABSTRACT A biocular viewing system for thermal (far infrared) viewers is provided by matching an eyepiece developed for an image intensifier to the low intensity image produced by light emitting diodes. A simplified image intensifier with gain sacrificed provides a low cost matching unit.

5 Claims, 13 Drawing Figures US. Patent N0v.25, 1975 Sheet10f6 3,922,524

\2 H i MONOCULAR F/G. la

BINOCULAR BIOCULAR U.S. Patent Nov. 25, 1975 Sheet 2 Of6 3,922,524

EYEPIECE APERTURE FIG. 4

U.S. Patfint Nov. 25, 1975 Sheet 3 of 6 3,922,524

SPATIAL FREQUENCY IN LINE PAIRS/mm FIG. 5

FIG. 6

FIG. 8

FIG. 7

U.S. Patent Nov. 25, 1975 Sheet50f6 3,922,524

US. Patent mA/WATT Nov. 25, 1975 Sheet 6 of 6 3,922,524

LED X 0 QUANTUM EFFICIENCY I% QUANTUM EFFICIENCY QUANTUM EFFICIENCY 4000 B000 IOOOO ANGSTROMS S -20 PHOTOCATHODE THIS SEMI-TRANSPARENT PHOTO-EMITTER CONVERTS LIGHT PROTONS INTO PHOTO-ELECTRONS SPATIAL FREQUENCY IN LINE PAIRS lmm DIODE TUBE MTF (ITT SER. NO. D4]

MODULAR BIOCULAR EYEPIECE FOR THERMAL IMAGE SYSTEMS This is a division of application Ser. No. 399,016, filed Sept. 20, 1973, now U.S. Pat. No. 3,868,504.

BACKGROUND OF THE INVENTION Covert optical surveillance and detection systems principally for night operations is a rapidly expanding business. The best known work is related to military applications, but there are many applications in police activities, hunting, wild life studies and commercial operations.

These systems can be divided roughly into two types. The first use image intensifiers for the visible and near visible. and the second operate in the far-infrared. Image intensifier systems which utilize incident visible light enhanced by nearby ultraviolet and infrared are in a high state of development. These devices range from a small bandheld type to ones with huge objective lenses that must be mounted on tripods. These devices must cover such a large field of interest that they must be viewed critically over long periods of time to efficiently utilize the information afforded. The display is the rather brilliant variety used in radar equipment and television emphasizing the green spectrum to which the eye is most sensitive. Monocular eyepieces were supplied in the original units and there were later replaced with biocular eyepieces. The latter eyepiece is so successful in reducing eye fatigue it may entirely replace the monocular type on large units, if its higher cost can be reduced sufficiently by mass production and value engineering.

The far infrared viewer is another night vision aid which also has unique daylight uses. These devices produce images based on the temperature difference in objects under surveillant and furnish information not perceived by the human eye under any ambient light condition. The state-of-the-art of thermal detectors, however, is not suited to intensifier tube direct view approaches. The most promising technique at present involves detector diodes and light emitting diodes in coupled pairs, with a scanning system to reduce the number of diodes, since these are still relatively expensive. Unfortunately, light emitting diode arrays do not have nearly the brightness of an image intensifier tube so that the eyepiece components of the two are not interchangeable. Monocular viewing of the thermal raster presentation at the low light level is extremely fatiguing to the human eye.

SUMMARY OF THE INVENTION The present invention provides a solution to the above problem, by forming a compound system including both light emitting diodes and a novel image intensifier tube. The tube and its adapter fitting are shaped like a wafer to minimize the slight change in the length of the optical path. The adapter fitting accepts lenses designed for image intensifier tubes, which includes biocular types.

BRIEF DESCRIPITON OF DRAWINGS The invention is best understood with reference to the accompanying drawings wherein:

FIG. 1 shows a ray diagram of three basic viewing systems;

FIG. 2 shows a side view ofthe elements which make up a biocular eyepiece designed for use with an image intensifier tube;

FIG. 3 shows a ray diagram of an image intensifier tube and a biocular eyepiece;

FIG. 4 shows a field of view overlap diagram for a biocular eyepiece;

FIG. 5 shows a graph of the Modulation Transfer Function for a biocular eyepiece;

FIG. 6 shows a schematic diagram of a typical far infrared viewing system;

FIG. 7 shows a ray diagram (unfolded) of the visual portion of the system from FIG. 6;

FIG. 8 shows a simplified ray diagram of the visual portion of the system from FIG. 6 with a biocular eyepiece added;

FIG. 9 shows the ray pattern resulting when an image intensifier tube is placed with its input face at the reticle plane in FIG. 8;

FIG. 10 shows a graph of the spectral response for an S- 20 cathode with the spectral output of an LED superposed thereon; and

FIG. II shows a graph of the Modulation Transfer .function for the matching image intensifier tube according to the present invention.

DESCRIPTION OF INVENTION Referring to FIG. 1 the three basic systems of viewing an illuminated image with an eyepiece system are shown. The monocular system with one lens ll, FIG. Ia, puts the stress entirely on one eye I2 while the other eye is dark adapted. Shifting from one eye to the other thus induces additional stress. The binocular system of FIG. 1b, using lenses I3 and 14 for both the right and left eye 15 and 16, is better in terms of eye fatigue, but is still an uncomfortable arrangement that must be readjusted to the interpupillary distance of each observer. The biocular system of FIG. 10, using a single large lens 15 with an exit pupil large enough for both eyes I6 and 17 requires no adjustment for different observers and with adequate eye relief is very comfortable. On occasion two viewers have used such a system without difficulty.

FIG. 2 shows a suitable commercially available biocular eyepiece. It has five elements 2 l-25 which provide color correction for a green phosphor peaked at 5500 Angstroms. It is designed to image the 25mm diameter circular screen of an image intensifier tube with a 23 field of view. The lens has a speed of f/0.55. The exit pupil and eye relief are both 64mm. Distortion (barrel) is 4.4%. These and other characteristics of the lens are listed in Table I.

FIG. 3 shows the eyepipece 15 positioned in front of an image intensifier 31 with phosphor screen 32. The focus at minus 2.5 diopters. A virtual image appears l5.8 inches from the eyes 34 and 35 (normal reading distance).

FIG. 4 illustrates the overlap of fields 41 and 42 presented to a eye. Since complete overlap requires an extremely large lens a tradeoff is customarily made between complete viewer contact and reasonable lens size. The lens system specified in Table I has approximately percent overlap which still provides comfortable viewing for the operator.

FIG. 5 shows the image quality of the lens system in terms of the modulation transfer function. A 5mm stop is used to stimulate the photopic eye aperture. The function is shown on axis, curve 52; 14mm off axis,

3 curve 53', and 32 mm of axis, curve 54; compared with the maximum value permitted by the diffraction oimit curve The actual off axis operation is of course determined by viewers eye separation, i.e.. somewhere between curves S3 and S4.

A typical infrared scanning system with which the lens is used is shown in FIG. 6. An IR objective lens 61 forms a thermal image on an array of detector diodes after reflection from a scan mirror 62. The low frequency signals from the detectors are each amplified by a separate amplifier 69 and drive a light emitting diode (LED) 64. The one dimensional light image of the latter diodes is redirected by just mirror 65 to the back surface of the scanning mirror 62 which is treated to reflect visible light. A second mirror 68 redirects the two dimensional visible image formed by the scanning mirror to an eyepiece l5. Lenses 66 and 67 collimate the light reflected by the scanning mirror to reduce distortion.

FIG. 7 shows the visual optical relay from FIG. 6 in unfolded form. Elements 7] and 72 makeup lens 66 from FIG. 6 and elements 7376 makeup lens 67. The light output of the LEDs can be as low as 0.3 of a footlambert. To conserve size and weight an optically slow relay system (f/9.0) is used in the monocular system. Planar element 77 is a reticle plate.

FIG. 8 shows the result of adding an existing fast biocular lens (0.55) in place of the normal monocular eyepiece. The entire optical relay from FIG. 7 is represented by lens 81. A real image is formed at reticle plane 82. The maximum separation of observable images at the viewing plane 83 is much less than the viewers eye separation, so that biocular viewing is impossible.

FIG. 9 shows a ray diagram of the system of FIG. 8 beginning at the reticle plane after an image intensifier tube 91 has been placed with its input face at the reticle plane, and the biocular eyepiece 15 is focused on its output face to provide the desired exit pupil/eye relief (see Table I). Other methods of correcting the optical mismatch shown in FIG. 8 are (l) to drive the LEDs harder and place a diffusing screen in the reticle plane (2) to increase the speed (i.e., lower the f/number) of the visual relay or (3) to substitute a cathode ray tube (CRT) display for the LEDs. LEDs are not yet available which will provide a factor of 100 increase in brightness as required by method I without seriously degrading their lifetime. Lowering the f/number requires an increase of size and weight that is intolerable for a portable system of this type. The use of a cathode ray tube requires complete redesign of the electronics to include a multiplexing system. Retrofitting existing units by this method would be tedious and costly.

Any existing image intensifier tube with adequate display area can be used for element 91 in FIG. 9, but a novel tube of simple design has been devised especially for this application. The tube consists of two fiber optic backing plates or face plates 92 and 93 which provide better defined resolution of the images passing therethrough. These plates are commercially available and commonly used in intensifier tubes. A photocathode consisting, for example. of 5-20 material (SbKNaCs) is deposited on the inner face of plate 92 and a phosphor screen which may be, for example. a P-ZO material (ZnCdSiAg) is deposited on the inner surface of plate 93. The two plates are positioned so that the cathode and phosphor screen are in close proximity, about one millimeter apart. An annular closure and sealing memher 94 is attached to the edges of the plates to maintain their spacing and permit evacuation of the space therebetween to approximately 10* torr. Both the cathode and anode (phosphor screen) are provided with thin conductive layers which pass high energy electrons. but act as light reflectors and ion barriers. Leads 99 connected these layers with external electrodes insulated from one another and sealed through the annular closure member 94. An adapter ring 99 surrounds closure member providing female threads 100 at one end and male 101 threads at the opposite end so that it can easily be interposed between the infrared viewer and the biocular lens. The adapter and tube may both be threaded to engage one another or the adapter may consist of two sections secured by mating threads 102 for easy tube replacement. During operation on a voltage of approximately 8kv is maintained across the electrodes. The external faces of the tube are spaced only I? mm apart thus adding very little to the length of the optical pattern.

FIG. 10 shows the conversion efficiency of 5-20 cathode with the energy spectrum of a GaAs LED superposed. The excellent matching of the two is evident. Newer cathodes with different response characteristics are available, if the specifications of any far infrared viewer should become critical in this regard.

FIG. 11 shows a graph of the modulation transfer characteristic of the tube shown in FIG. 9. The function falls off less rapidly than that of most conventional intensifier tubes and compares favorably with the function for passive elements like the biocular lens of FIG. 5. Other characteristics of the tube are listed in Table II.

Obviously many variations of the above described structures will be immediately apparent to those skilled in the art, but the present invention is limited only to the extent defined in the claims which follow.

TABLE I BIOCULAR CHARACTERISTICS 1. In an infrared viewing system wherein an infrared image is converted to a low intensity visible image for direct monocular viewing; the improvement comprising:

a thin photocathode layer of 8-20 type photoemissive material is attached to the opposite broad surface of said plate.

3. The system according to claim I wherein:

the gain of said intensifier is approximately I00 footlamberts/footcandle.

4. The system according to claim I wherein:

the thickness of said intensifier is approximately 17 millimeters.

5. The system according to claiml wherein the exit pupil of said eyepiece is approximately 64mm. 

1. In an infrared viewing system wherein an infrared image is converted to a low intensity visible image for direct monocular viewing; the improvement comprising: a thin flat low-gain image intensifier mounted in said system with its input face substantially in the plane of said visible image; and a biocular eyepiece having a focal length less than its diameter adjustably mounted at approximately said focal length distance from the output face of said intensifier.
 2. The system according to claim 1 wherein: said visible image is generated by light emitting diodes; said input face is one broad surface of a first thin fiber optic face plate; and a thin photocathode layer of S-20 type photoemissive material is attached to the opposite broad surface of said plate.
 3. The system according to claim 1 wherein: the gain of said intensifier is approximately 100 foot-lamberts/footcandle.
 4. The system according to claim 1 wherein: the thickness of said intensifier is approximately 17 millimeters.
 5. The system according to claim 1 wherein the exit pupil of said eyepiece is approximately 64mm. 